Organic cell

ABSTRACT

Lives of organic secondary cells having organic electrodes wherein organic conductive materials such as polyacetylene are used as electroactive substance for the cathodes and/or anodes can be extended in the following way: Active species which are produced, when organic electrodes are doped with excess ions, and are responsible for the deterioration of cell functions are trapped with a substance having a frontier-electron density higher than those of the organic conductive materials, such as dimethylimidazolidinone, toluene, diphenylamine and dimethylaniline, by adding the substance to electrolyte, such as a solution of lithium perchlorate in propylene carbonate, of the cells.

BACKGROUND OF THE INVENTION

The present invention relates to organic secondary cells having organicelectrodes wherein organic conductive materials such as polyacetyleneare used as electroactive substance for the positive electrodes(cathodes) and/or the negative electrodes (anodes). More particularly,the invention relates to improvements in electrolytes for use in organicsecondary cells.

Organic secondary cells operate in the manner so that the organicelectrodes during charge are doped with ions of the electrolyte whichconsists of, for example, a solution of lithium perchlorate in propylenecarbonate, and the doped ions during discharge effuse into theelectrolyte to cause an electric current through the external circuit.However, when the organic electrodes are doped with excess of such ionsby overcharge or the like, these ions dissociate into active species,for example, free radicals, which will react with the organic electrodesto lower the conductivity thereof, thus deteriorating cell functions.

SUMMARY OF THE INVENTION

An object of the invention is to extend lives of organic cells throughpreventing the organic electrodes of the cells from reacting with theabove-noted active species which are responsible for the deteriorationof cell functions, by adding a substance which will trap such activespecies, to the electrolyte (hereinafter the original electrolyte isreferred to as the mother electrolyte).

According to the invention, there is provided an electrolyte for organicsecondary cells having organic electrodes wherein organic conductivematerials are used as electroactive substance for the positiveelectrodes and/or the negative electrodes, which comprises a motherelectrolyte and an additive which has a frontier-electron density higherthan those of the organic conductive materials.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing the structure of an organicsecondary cell pertaining to the invention.

FIG. 2 shows frontier-electron densities of organic compounds includingadditives for use in the electrolyte of the invention.

FIG. 3 is a graph showing the relation between the charge-dischargeefficiency (%) and the number of charge-discharge cycles determined onorganic cells pertaining to the invention.

FIG. 4 is a graph showing the relation between the life of organic cellspertaining to the invention and the additive content (vol %) inelectrolyte of the cells, where the life is represented by the number ofcharge-discharge cycles repreated before the charge-discharge efficiencydrops below 40%. In this figure, curves 1 and 2 were obtained whendimethylimidazolidinone and toluene were used as additives,respectively.

DETAILED DESCRIPTION OF THE INVENTION

As a result of infrared absorption spectroscopy and surface analysis,the present inventors found out that active species produced byexcessive ion doping of an organic electrode are liable to react withπ-bonds of the organic conductive electrode material, and invented amethod for preventing the reaction, which comprises the addition to themother electrolyte a substance higher in π-electron density, morespecifically in frontier electron density, than the organic conductivematerial.

According to the invention, the reaction of active species with theorganic conductive material can be prevented, since the additive is moreliable to react with those active species than the organic conductivematerial, and in consequence the drop in the conductivity thereof can beprevented and the cell life can be improved to a great extent.

Referring now to the drawings, the invention is described in detail.

In FIG. 1, the numeral 1 represents outgoing electrodes (terminals)consisting of a conductor, such as a metal, e.g. Ni, Al, or Ti or aninorganic polymer, e.g. carbon fiber, which does not dissolve in theelectrolyte 6 during charge or discharge. The conductor may be either asimple substance or a composite like a conductive rubber. The numeral 2represents a cathode and an anode, at least one of which consists of anorganic conductive material such as polyacetylene or polypyrrole. Otherorganic conductive materials such as poly(p-phenylene) and polythiophenealso can be used as the electrode material. When such an organicconductive material is used for only one of the electrodes, the otherelectrode may consist of an inorganic conductive material inert to theelectrolyte 6. Such materials include metals, e.g. Ni, Al, and Pt andother inorganic conductors, e.g. graphite and carbon fiber. At least oneside of the organic electrode can be covered with a thin layer of metalaccording to the method for making electrodes of organic cells describedin Japanese Pat. Application No. 120,770/82, which was filed by thepresent applicant. This method comprises forming a thin layer of metalon at least a part of at least one side of an organic electrode at atemperature where the organic electrode does not degrade, and thenforming an atomic mutual diffusion layer between the thin metal layerand the organic electrode sheet by the recoil-ion implantation method.The numeral 3 represents plugs for sealing the holes through which theoutgoing electrodes penetrate. These plugs may consist of any ofmaterials including the same one as used for the cell case 5 which willbe stated below, other resins, and metals, provided that the materialcan maintain the complete air-tightness as well as the insulation of thetwo electrodes from each other. The numeral 4 represents a separatorthat prevents both electrodes from touching each other and is permeableto ions of the electrolyte 6. The separator 4 consists of a nonwovenfabric of polypropylene, a glass filter, or the like. The cell case 5may consist of any material inert to the electrolyte 6. Suitablematerials for the cell case 5 include insulators, e.g. polypropylene andTeflon, of course, and also conductors, e.g. aluminum and stainlesssteel provided that the insulation from the outgoing electrodes issecured. The electrolyte 6 consists of a mother electrolyte and anadditive. The mother electrolyte is a solution of electrolyte in anonaqueous solvent. In the following Example, a solution of lithiumperchlorate in propylene carbonate was used as the mother electrolyte.Electrolyte usable herein also include other compounds such as lithiumborofluoride, which do not react with the organic electrode anddissociate into ions in the nonaqueous solvent. While propylenecarbonate was used as the nonaqueous solvent in the following Example,other solvents such as γ-butyrolactone and tetrahydrofuran can be usedas well.

The additive to be incorporated in the mother electrolyte in theinvention is an electron donative substance or anelectron-donative-group containing substance and has a frontier-electrondensity higher than that of the organic electrode material. Examples ofthe additive are dimethylimidazolidinone, toluene, diphenylamine, anddimethylaniline, frontier-electron densities of which are shown in FIG.2.

When polyacetylene is used as the organic conductor for an electrode,dimethylimidazolidinone, toluene, diphenylamine, and dimethylaniline,which have a frontier-electron density of higher than 0.35 are suited asthe additive since the frontier-electron density of polyacetylene is0.35. Other substances can also be used as the additive, provided thatthey have frontier-electron density higher than that of the organicelectrode material used. The additive may consist of either onecomponent or a mixture of plural components.

The invention is illustrated in more detail referring to the followingExample:

EXAMPLE

An electrolyte was prepared by adding 1 vol % ofdimethylimidazolidinone, highest in frontier-electron density among thecompounds shown in FIG. 2, to a 1 M solution (mother electrolyte) oflithium perchlorate in propylene carbonate. An organic cell wasconstructed using the above prepared solution as the electrolyte,polyacetylene for the cathode, and aluminum for the anode. Forcomparison, similar organic cells were constructed without using anyadditive and with using as additive 1 vol % of carbon tetrachloridewhich accelerates the reaction of active species with polyacetylene,respectively.

These organic cells were subjected to an accelerated endurance test,wherein charging and discharging conditions were as follows: Each cellwas charged at a constant current density of 1 mA/cm² to a chargequantity of 4 C/cm² and then discharged through a resistance of 110 Ω toa voltage of 1 V. This process corresponds to one charge-dischargecycle.

The results were as shown in FIG. 3, wherein the number on the ordinateindicates the charge-discharge efficiency (%) and the number on theabscissa indicates the number of charge-discharge cycles. In the figure,curve is of the case where dimethylimidazolidinone was added, curve ofthe case where no additive was used, and curve of the case where carbontetrachloride was added. The results showed that the life of the cellcontaining 1 vol % of dimethylimidazolidinone as additive to the motherelectrolyte was 3 times as long as that of the control cell (containingno additive). Also when other additives toluene, diphenylamine, anddimethylaniline were incorporated respectively, lives of the cells weremuch extended as compared with the control cell. The suitable amount ofadditive cannot be absolutely specified since it of course varies withthe kinds of mother electrolyte used and of additive itself. However,when a 1 M solution of LiClO₄ in propylene carbonate as the motherelectrolyte and polyacetylene as the organic electrode material areused, suitable amounts of additives are up to 10% by volume as shown inFIG. 4. In this figure, the number on the ordinate indicates a measureof cell life, that is, the number of charge-discharge cycles repeatedbefore the charge-discharge efficiency drops below 40%, and the numberon the abscissa indicates the amount (vol %) of additive incorporated.Curves 1 and 2 were obtained when dimethylimidazolidinone and toluenewere used as additives, respectively.

What is claimed is:
 1. An electrolyte for organic secondary cells havingorganic electrodes wherein organic conductive materials are used as theelectroactive substance for the cathodes and/or the anodes, whichelectrolyte consists essentially of a solution of lithium perchlorate ina non-aqueous propylene carbonate solvent as a mother electrolyte anddimethylimidazolidinone as a cosolvent additive which has afrontier-electron density higher then those of the organic conductivematerials, the dimethylinidazolidinone additive preventing the reactionof active species formed by dissociation of the mother electrolyte withthe organic conductive material of the organic electrodes.